In the past, dynamoelectric machines, such as an electric motor or the like, were provided with various different types of housings, such as for instance a rolled steel housing or a cast metallic housing or the like, and each such housing had generally axially extending radially spaced apart inner and outer circumferential surfaces with a pair of generally opposite end faces interposed therebetween, respectively. During the assembly of the past electric motors, a stator assembly including windings was mounted to the housing within its inner circumferential surface by suitable means, such as for instance pressing the stator assembly into a press-fit engagement with the inner circumferential surface. When the stator assembly was press-fitted into a rolled steel housing, it is believed that distortion occurred which necessitated corrective machining for concentricity purposes; however, when the stator assembly was press-fitted into a cast housing, it is believed that such corrective machining thereof was necessary for concentricity purposes whether or not the cast housing was deformed. Therefore, past practice was to chuck the stator assembly on a bore extending therethrough and then machine opposite internal rabbets in the inner circumferential surface of the housing or opposite external rabbets in the outer circumferential surface of the housing with the opposite rabbets intersecting the opposite end faces on the housing. When internal rabbets were machined into the housing, it is believed that some of the metal chips formed during the machining of the internal rabbets in the inner circumferential surface of the housing may have lodged in the windings of the stator assembly, and it is also believed that such lodged chips may have contributed to short circuiting of such windings.
Further with respect to the past practices, various national and international regulatory agencies, such as NEMA and Underwriters Laboratories, Inc. in the U.S.A. and the International Electrical Commission (hereinafter referred to as IEC) for instance, have established dimensional and electrical standards for different types of electrical apparatus. In the field of electric motors, both NEMA and IEC have established standard frame sizes with each such frame size having certain standardized dimensions and with the IEC standardized dimensions being on the metric scale. For instance, an electric motor of any given frame size would have at least the following standardized dimensions: a "shaft height" dimension, i.e., the vertical distance between the centerline axis of the motor shaft and the base or seating surface of the motor mounting pads; an "A" dimension, i.e., the horizontal distance between the centerline axes of the mounting bolt holes in the opposite motor mounting pads; and a "B" dimension, i.e., the horizontal distance between the centerline axes of the mounting bolt holes in either one of the mounting pads. Furthermore, the mounting bolts utilized to mount electric motors of a given frame size have also been provided by NEMA and IEC with standardized diameter dimensions.
Of course, the standardization by NEMA of the mounting bolt diameters and the "A" and "B" dimensions for given frame sizes of electric motors was advantageous from the viewpoint of interchanging electric motors of different electric motor manufacturers. For instance, the aforementioned standardization of the mounting bolt diameters and the "A" and "B" dimensions made it possible for a user in the U.S.A. of electric motors to provide identical mounting arrangements to receive or mount an electric motor of a given frame size manufactured by many different domestic electric motor manufacturers. Of course, the same may be said for a user and manufacturers of electric motors in various other countries which have adapted the IEC standardized dimensions.
In order to maximize the horsepower rating of an electric motor of a given frame size, it is advantageous to provide the electric motor housing with an outside diameter which is as large as possible and which is limited by the aforementioned standardized shaft height dimension for such given frame size. Of course, the greater the outside diameter, the greater the inside diameter thereby to accept a larger diameter stator which results in an increased horsepower rating. Of course, the horsepower rating may also be increased by "stacking-out" the stator, i.e., increasing the axial length of the stator; however, when a "symmetrical" electric motor is desired, it is well known to the art that the stator cannot be "stacked-out".
While it is advantageous to increase the outside diameter of the housing for an electric motor of a given frame size, as discussed above, it is believed that mounting problems may have been encountered when external mounting flanges were formed on either such housing or end frames for securement thereto. For instance, it is believed that the aforementioned larger outside diameter of the electric motor housing and the location of the external mounting flanges may have deleteriously interfered with the insertion of a standard mounting bolt through mounting holes provided on the aforementioned standardized "A" and "B" dimensions in the mounting pads of such electric motors.